1. Field of the Invention
The present invention relates to a surface acoustic wave device which operates in a high frequency range of from several hundred MHz to GHz band.
2. Description of the Related Art
Since a surface acoustic wave device utilizing a surface acoustic wave which propagates with energy concentrated on a surface of a solid material is a compact and stable device, it is practically used as an intermediate frequency filter in a TV set and the like.
The surface acoustic wave device is usually excited by applying an alternating electric field on a piezoelectric material with comb-like electrodes.
As the piezoelectric material, bulk single crystals such as LiNbO.sub.3, LiTaO.sub.3, etc. or a ZnO thin film which is vapor deposited on a substrate is used. Currently, a surface acoustic wave device which comprises a single crystal piezoelectric material, or a ZnO piezoelectric material formed on a glass plate or a sapphire plate is practically used.
In general, an operation frequency f of the surface acoustic wave device is determined by the equation: f=v/.lambda. in which is a propagation velocity of the surface acoustic wave and .lambda. is a wavelength. The wavelength .lambda. is determined from a period of the comb-like electrode as shown in FIGS. 1 and 2. In FIG. 1, electrode tips each having a width d are integrally formed with a distance 3d. A pair of adjacent electrode tips are of the different electrodes and every other electrode tips are of the same electrodes. The comb-like electrode of FIG. 1 is one of the most commonly used electrodes. The wavelength with this type of electrodes is 4d.
In the comb-like electrode of FIG. 2, two electrode tips each having a width d are repeatedly arranged with a distance 5d. The wavelength .lambda. is 8d/3. With the comb-like electrode of FIG. 2, a three-times mode is strongly excited.
The propagation velocity v depends on the piezoelectric material or the substrate material and also on the mode of surface acoustic wave.
When the single crystal piezoelectric material made of LiNbO.sub.3 is used, the propagation velocity v is from 3500 to 4000 m/sec., and when that made of LiTaO.sub.3 is used, the propagation velocity v is from 3300 to 3400 m/sec. When the piezoelectric material comprising the ZnO thin film formed on the glass plate is used, the propagation velocity v is at most 3000 m/sec.
To increase the operation frequency f, the propagation velocity v is increased and/or the wavelength .lambda. is decreased. However, the propagation velocity v is limited by the characteristics of the material. The period size of the comb-like electrode has a lower limit due to limitation of fine processing technique. By the photolithography, the lower limit of the period size is 1.2 .mu.m. With the electron beam exposure, the processing to submicron order is possible. However, the line width becomes smaller, an yield becomes worse. That is, because of the limitation of the processing technique, the wavelength .lambda. cannot be reduced significantly.
By the above reasons, the operation frequency of the practically used surface acoustic wave device is at most 900 MHz, and one having the operation frequency larger than 900 MHz has not been widely produced.
By the way, as the frequency in the telecommunication such as satellite telecommunication or mobile telecommunication is increased, it is increasingly required to provide a surface acoustic wave device which can be used in a high frequency range (GHz band), and development of such surface acoustic wave device is being made vigorously.
In general, to apply the piezoelectric thin film grown on the substrate in the surface acoustic wave device, when the sound velocity through the substrate is larger than that through the piezoelectric material, plural surface acoustic waves having different propagation velocities (zeroth order mode, first order mode, second order mode and so on from the wave having the smaller propagation velocity) are generated.
When the sound velocity through the substrate material is larger, the propagation velocity v becomes larger.
Of course, the propagation velocity v of the surface acoustic wave and the sound velocity through the substrate material are in the different concepts and have different values. However, since the thin piezoelectric layer is formed on the substrate, the surface acoustic wave which propagates on the piezoelectric layer is strongly influenced by elasticity of the substrate. Therefore, when the sound velocity through the substrate material is large, the propagation velocity of the surface acoustic wave is large.
Then, a prototype device comprising a substrate of sapphire through which the sound velocity is large (a velocity of transversal wave: 6000 m/sec., a velocity of longitudinal wave: 12,000 m/sec.) and a ZnO piezoelectric thin film formed thereon was produced. This prototype device achieved the propagation velocity of 5500 m/sec.
Since the sound through diamond has the largest velocity, a surface acoustic wave device comprising a diamond substrate will realize a propagation velocity of 10,000 m/sec. or larger. Since the sound velocity through a diamond-like carbon is substantially the same as that through the diamond, a device comprising the diamond-like carbon substrate will realize the same propagation velocity as in case of the device comprising the diamond substrate. Such device is described in Japanese Patent Application No. 20635/1989.
To provide a surface acoustic wave device, first, an electromechanical coupling factor K.sup.2 which is a measure for a conversion efficiency in converting electric energy to mechanical energy should be large. Usually, K.sup.2 should be 0.5% or larger. Second, to provide a surface acoustic wave device having a high operation frequency, the propagation velocity v should be large.
When the thin piezoelectric film formed on the substrate is used, the propagation velocity v and the electromechanical coupling factor K.sup.2 greatly depend not only on the physical properties of the piezoelectric material and substrate material but also on a film thickness H1 of the piezoelectric film.
When the substrate is in a thin film form, namely the device has a laminate structure of piezoelectric film/substrate film, they depend on a film thickness H2 of the substrate film, too.
However, hitherto, a relationship of the propagation velocity v and the electromechanical coupling factor K.sup.2 with the film thicknesses H1 and H2 of the piezoelectric film and the substrate film has not been clarified, when the substrate film is made of diamond.
Therefore, a practical surface acoustic wave device comprising a diamond substrate has not been produced.